I found this helpful illustration from the BBC via Warren Ellis. For those of you who haven't heard, the International Astronomical Union—them folks what gets to name space-y stuff—may have finally settled the issue of whether or not Pluto was actually a planet, what with its miniscule size and weirdo sometimes-I'm-closer-than-Neptune-sometimes-I'm-not orbit. Officially, Pluto is a planet, but any specific definition of a planet that would allow for Pluto also automatically qualifies a number of the larger, less well known objects orbiting our favorite sun.
To quote the Wired article: "The panel also proposed a new category of planets called 'plutons,'
referring to Pluto-like objects that reside in the Kuiper Belt, a
mysterious, disc-shaped zone beyond Neptune containing thousands of
comets and planetary objects. Pluto itself and two of the potential
newcomers Charon and 2003 UB313 would be plutons."
Don't completely rewrite the textbooks just yet, as my Jedi Master John Scalzi has a cogent prediction of future planetary classification: "what I think will eventually happen is that there will be nine 'Historical Planets' that get named in popular astronomy books, with
Pluto/Charon being considered one entry ... and then all the other planets get a
hand-wave, as in: 'Our solar system is comprised of nine historical
planets, and many other smaller, icy planets discovered after Pluto.'
Done and done. Among other things, this will allow people not to worry
about screwing up the 'naming the planets after Roman gods' thing."
Me, I'm for rewriting text books, if only because that means I get to sell a whole new set of trivia questions to the world.